Apparatus and method for measurement of the thermal performance of an electrostatic wafer chuck

ABSTRACT

An apparatus and method are described for measuring the thermal performance of a wafer chuck, such as an electrostatic chuck. In one example, the apparatus has a chamber, a base to support a wafer chuck in the chamber, a heater to heat the chuck, a window through the exterior of the chamber, and an infrared imaging system to measure the temperature of the chuck while the chuck is heated.

TECHNICAL FIELD

Embodiments of the present invention relate to the microelectronicsmanufacturing industry and more particularly to temperature controlledchucks for supporting a workpiece during plasma processing.

BACKGROUND

In the manufacture of semiconductor chips a silicon wafer or othersubstrate is exposed to a variety of different processes in differentprocessing chambers. The chambers may expose the wafer to plasmas,chemical vapors, metals, laser etching, and various deposition and acidetching processes in order to form circuitry and other structures on thewafer. During these processes, the silicon wafer may be held in place byan electrostatic chuck (ESC). The chuck holds the wafer by generating anelectrostatic field to clamp the back side of the wafer to a flatsurface or puck surface of the chuck.

As fabrication techniques for plasma processing equipment advance, suchas those designed to perform plasma etching of microelectronic devicesand the like, the temperature of the wafer during processing becomesmore important.

ESCs have been designed for thermal uniformity across the surface of thesubstrate, sometimes called a workpiece. ESCs use liquid cooling toabsorb the plasma power heat and remove it from the chuck. An ESC mayalso include independently controlled heaters in multiple zones. Thisallows for a wider process window under different process and plasmaconditions. Individual heater zones in the radial direction can createvarious radial temperature profiles which compensate for other etchprocess radial non-uniformities.

In semi-conductor etch processing, the temperature of a wafer duringprocessing influences the rate at which structures on the wafer areetched. Other processes may also have a temperature dependence. Thistemperature influence is present, for example, in conductor etchapplications in which very precise wafer temperature control helps toobtain a uniform etch rate. A more precise thermal performance allowsfor more precisely formed structures on the wafer. Uniform etch ratesacross the wafer allow smaller structures to be formed on the wafer.Thermal performance or temperature measurement and control is thereforea factor in reducing the size of transistors and other structures on asilicon chip.

SUMMARY

An apparatus and method are described for measuring the thermalperformance of a wafer chuck, such as an electrostatic chuck. In oneexample, the apparatus ha a chamber, a base to support a wafer chuck inthe chamber, a heater to heat the chuck, a window through the exteriorof the chamber, and an infrared imaging system to measure thetemperature of the chuck while the chuck is heated.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example,and not limitation, in the figures of the accompanying drawings inwhich:

FIG. 1 is a schematic of a thermal performance measurement systemincluding a chuck assembly in accordance with an embodiment of thepresent invention;

FIG. 2 is a simplified cross-sectional diagram of a portion of anelectrostatic chuck in accordance with an embodiment of the invention;

FIG. 3 is a simplified cross-sectional diagram of a model of ESCtemperature and thermal conduction for an electrostatic chuck inaccordance with an embodiment of the invention;

FIG. 4 is a diagram of a top elevation contour line graph of thetemperature of a wafer in a processing chamber;

FIG. 5 is an isometric view of an electrostatic chuck in accordance withan embodiment of the invention;

FIG. 6 is an isometric diagram partially cut away of a thermalperformance measurement chamber with shutters closed in accordance withan embodiment of the invention.

FIG. 7 is an isometric diagram partially cut away of a thermalperformance measurement chamber with shutters open in accordance with anembodiment of the invention.

FIG. 8 is an isometric view of an annular heater with wire filaments inaccordance with an embodiment of the invention.

FIG. 9 is an isometric view of an annular case heater with concentriccase heater elements in accordance with an embodiment of the invention.

FIG. 10 is a process flow diagram of measuring the thermal performanceof a chuck in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

In the following description, numerous details are set forth, however,it will be apparent to one skilled in the art, that the presentinvention may be practiced without these specific details. In someinstances, well-known methods and devices are shown in block diagramform, rather than in detail, to avoid obscuring the present invention.Reference throughout this specification to “an embodiment” or “oneembodiment” means that a particular feature, structure, function, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention. Thus, the appearances ofthe phrase “in an embodiment” or “in one embodiment” in various placesthroughout this specification are not necessarily referring to the sameembodiment of the invention. Furthermore, the particular features,structures, functions, or characteristics may be combined in anysuitable manner in one or more embodiments. For example, a firstembodiment may be combined with a second embodiment anywhere theparticular features, structures, functions, or characteristicsassociated with the two embodiments are not mutually exclusive.

As used in the description of the invention and the appended claims, thesingular forms “a”, “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willalso be understood that the term “and/or” as used herein refers to andencompasses any and all possible combinations of one or more of theassociated listed items.

The terms “coupled” and “connected,” along with their derivatives, maybe used herein to describe functional or structural relationshipsbetween components. It should be understood that these terms are notintended as synonyms for each other. Rather, in particular embodiments,“connected” may be used to indicate that two or more elements are indirect physical, optical, or electrical contact with each other.“Coupled” my be used to indicate that two or more elements are in eitherdirect or indirect (with other intervening elements between them)physical, optical, or electrical contact with each other, and/or thatthe two or more elements co-operate or interact with each other (e.g.,as in a cause an effect relationship).

The terms “over,” “under,” “between,” and “on” as used herein refer to arelative position of one component or material layer with respect toother components or layers where such physical relationships arenoteworthy. For example in the context of material layers, one layerdisposed over or under another layer may be directly in contact with theother layer or may have one or more intervening layers. Moreover, onelayer disposed between two layers may be directly in contact with thetwo layers or may have one or more intervening layers. In contrast, afirst layer “on” a second layer is in direct contact with that secondlayer. Similar distinctions are to be made in the context of componentassemblies.

The temperature uniformity across the surface of an ESC has beenimproved with improved cooling plate and heater designs and improvementsin bonding the cooling plate to the puck that holds the workpiece.However, these designs and processes are still subject to manufacturingvariations, which can lead to significant thermal non-uniformity. Forsome implementations, a spatial temperature variation of less than+/−0.3° C. across the wafer is desired.

FIG. 1 is a schematic of an electrostatic chuck testing platform 100that is able to mimic the thermal environment of a plasma etch system.The test system 100 includes a grounded chamber 105. Recharge gases,either ambient air, nitrogen or other gases are supplied from gassource(s) 129 connected to the chamber through a mass flow controller149 to the interior of the chamber 105. Chamber 105 is evacuated via anexhaust valve 151 connected to a high capacity vacuum pump stack 155.

A wafer 110 may be used to further characterize the performance of thechuck. If so the wafer may be loaded into the chamber and clamped to thechuck assembly 142 inside the chamber. The workpiece 110, such as asemiconductor wafer, may be any wafer, substrate, or other materialemployed in the semi-conductor processing art and the present inventionis not limited in this respect. The workpiece 110 is disposed on a topsurface of a dielectric layer 143 or puck of the chuck assembly that isdisposed over a cooling base assembly 144 of the chuck assembly. A clampelectrode (not shown) is embedded in the dielectric layer 143. Inparticular embodiments, the chuck assembly 142 may include differentheater zones, such as a center zone 141 and edge zones 199, each zone141, 199 may be independently controllable to the same or to differenttemperature set points. The chuck assembly 144 includes a base to carrythe chuck including the heaters, cooling system, and the puck. A varietyof different chucks may be placed on the base of the chuck assembly fortesting purposes.

A system controller 170 is coupled to a variety of different systems tocontrol a chuck testing process in the chamber. The controller 170 mayinclude a temperature controller 175 to execute temperature controlalgorithms (e.g., temperature feedback control) and may be eithersoftware or hardware or a combination of both software and hardware. Thesystem controller 170 also includes a central processing unit 172,memory 173 and input/output interface 174. The temperature controller175 is to output control signals affecting the rate of heat transferbetween the chuck assembly 142 and a heat source and/or heat sinkexternal to the chamber 105 for the various heater zones 141, 199.

In embodiments, in addition to the different heaters, there may bedifferent coolant temperature zones. The coolant zones may includeseparate, independently controlled heat transfer fluid loops withseparate flow control that is controlled based on a zone-specifictemperature feedback loop. In the example embodiment, the temperaturecontroller 175 is coupled to a first heat exchanger (HTX)/chiller 177and may further be coupled to a second HTX/chiller 178 and a thirdHTX/chiller 179. The flow rate of the heat transfer fluid or coolantthrough conduits in the chuck assembly 142 may also be controlled by theheat exchangers.

One or more valves 185, 186 (or other flow control devices) between theheat exchanger/chillers 177, 178, 179 and fluid conduits in the chuckassembly 142 may be controlled by the temperature controller 175 toindependently control a rate of flow of the heat transfer fluid to eachof the different cooling zones. The heat transfer fluid may be a liquid,such as, but not limited to deionized water/ethylene glycol, afluorinated coolant such as Fluorinert® from 3M or Galden® from SolvaySolexis, Inc. or any other suitable dielectric fluids such as thosecontaining perfluorinated inert polyethers. While the presentdescription describes the ESC in the context of a plasma processingchamber, the ESC described herein may be used in a variety of differentchambers and for a variety of different processes.

The controller also has a heater controller 161 that is coupled to acurrent source 162 to drive a heater 164 inside the chamber. The currentmay be coupled through a switch matrix 163 to independently controldifferent zones of the heater. The heater applies heat to the chuckthrough thermal radiation in order to simulate heat that might begenerated by a plasma source in an etch chamber. The heater controlleris also coupled to an array of shutters 166. The shutters are hinged atone end and driven by shafts 167 or electromagnetic coupling by exteriorrotary motors 165. The shutters open and close to allow radiant heat 122from the heater to be blocked during imaging or passed to simulate aplasma heat load. The quick cycling of the heat by the shutters furthersimulates the environment inside an etch chamber.

The test system also has a window 168 at the top of the chamber. Thewindow allows a thermal imaging camera 169 to observe the chuck 142 andthe wafer 140, if any, through a central hole 124 in the heater. Thetemperature profile across the surface of the chuck or wafer can beobserved, and measured at different chamber temperatures. Using thisapparatus, the quality and the performance of the chuck can bedetermined.

FIG. 2 is a simplified cross-sectional diagram of a portion of anelectrostatic chuck (ESC) 200. There are at least four components forregulating the temperature of the chuck surface and therefore thetemperature of a wafer (not shown) on the chuck. A cooling plate 201,typically made from a thermally conducting metal serves as a heat sink.The cooling plate is bonded to a dielectric puck 202 with a hightemperature adhesive 204, such as silicone. The puck is typicallyceramic but may alternatively be made with other materials. Electrodes(not shown) are embedded within the puck to generate an electrostaticfield with which to grip a workpiece, such as a silicon substrate.Resistive heater traces 203 are also embedded within the puck fortemperature control.

The cooling plate 201 also includes channels 205 for coolant. Coolant ispumped through the channels to absorb heat from the cooling plate andpumped to a heat exchanger to tool the fluid which is then recirculatedback to the cooling plate. The cooling place absorbs heat from theembedded heaters and the workpiece through the ceramic plate. Thetemperature uniformly depends on the quality of the ceramic puck 202,the elastomer bond 204, and the cooling plate channels 205. It alsodepends on how well heat is translated from the workpiece to the ceramicpuck. All of these factors are object to variations in manufacture anduse.

FIG. 3 is a simplified one-dimensional diagram of a model of ESCtemperature and thermal conduction. The diagram is presented using thesame components and reference numbers as in FIG. 2, however, only aportion of the FIG. 2 diagram is shown for reference. In this model, theceramic temperature, Tceramic, at a given location 220 is determined inpart by the “thermal resistances” of the cooling plate (R cooling plate)222 and the bond (R bond) 221. Heat is provided by the heater power (Qheater) 223 and removed by the cooling plate (T cooling plate 224) andby the coolant (T coolant 226). The thermal resistance is presented forpurposes of explanation. The components of FIG. 3 may be described asfollows:

Q heater 223: The heater power at a given point on the ESC surface isdetermined by the number of heater traces in the area, and theelectrical resistance of those heater traces. When the chuck is in use,heat is also applied by the plasma. For testing purposes, the heatersmay be used to simulate plasma processing or any other high temperatureprocessing or a different external or internal heat source may be used.If the heater traces produce sufficient heating, then the heater tracesmay be used. Rather than generating temperatures similar to those usedfor plasma processing, the heater traces may be used simply to generatea measureable heat flow from the ceramic puck 202 to the othercomponents.

R bond 221: The resistance of the bond is determined by the thermalconductivity of the bond material, the bond thickness, and the qualityof the bond connection both to the cooling plate and to the ceramicpuck.

T cooling plate 224: The temperature of the cooling plate is largelycontrolled by the conduction of heat from the ceramic puck through thebond and into the coolant. The flow of heat into the coolant at any onelocation 224 of the cooling plate is affected by at least twofactors: 1) the coolant temperature increases as it travels through thecooling plate so that the coolant at different locations of the coolingplate will be at different temperatures and 2) feedthrus and otherfeatures of the cooling plate constrain where the cooling channels canbe placed in the cooling plate so that some locations have more coolantflow then others.

R cooling plate 222: The thermal resistance of the cooling plate is acombined function of the local fluid heat transfer coefficient, thegeometry of the cooling plate, and the thermal conductivity of thecooling plate.

T coolant 226: The temperature of the coolant entering the cooling platemay be carefully controlled by a heat exchanger or chiller. However, asthe coolant travels through the cooling plate, its temperatureincreases. In a typical application, the coolant temperature may rise byup to 10° C. So the local coolant temperature at a given point on theESC varies greatly.

T ceramic: The temperature of the ESC ceramic at any one particularlocation 220 may be estimated using the relationship:

T ceramic=Q heater R bond+Q heater R cooling plate+T coolant.

This shows that to achieve a uniform temperature across the ceramicpuck, R cooling plate may be adjusted at each location to compensate forvariation of R bond, Q heater, and T coolant. Alternatively, another wayto achieve the most uniform ceramic temperature possible is to designthe heater traces (and hence Q heater) to compensate for the spatialvariations of the cooling plate temperature. In other words Q heater isadjusted based on variations in R cooling plate and T coolant.

In an ideal ESC design, the heater watt density will be matched toperfectly compensate for variations in the cooling plate temperature.The bond thickness is uniform. As a result, the ceramic temperature isuniform in every dimension. In any real manufactured ESC. the ceramictemperature is non-uniform due to several factors. First, the design ofthe heater traces may not be perfect. As a result an ideally uniformwatt density is difficult to achieve. Second, the heater traces aremanufactured or created using a screen printing process. Printing errorcauses the actual watt density to deviate still further from theimperfect values that were designed for the traces. Third, the bondthickness of the adhesive varies. As a result, a typical manufacturedESC does not have perfectly uniform bond heat resistance.

FIG. 4 is a diagram of a top elevation graph 400 of the temperature of awafer 404 on an ESC. This graph shows surface temperature as a functionof position on the wafer using contour lines 402. Such a diagram may begenerated based on a measurement of an actual ESC as it is heated by theconductive traces and cooled by coolant flowing through the channels ofthe cooling base. Such measurements may be made by heating a wafer andthen measuring the temperature at different positions using an infraredcamera. As shown, the temperature values shown on the surfacetemperature contour lines vary in a pattern that is related tomanufacturing and design features of the ESC. Contour lines are shownonly for integer temperature values in order to simplify the diagram.For an actual measurement, much higher accuracy, for example tenths of adegree may be desired.

In order to even out the temperatures and obtain a more uniformtemperature across the ESC, external adjustments can be made to thecoolant flow within the ESC. In one example, the cooling plate containsmany (e.g. 50+) small flow adjustable orifices. These orifices may beadjusted by mechanically changing their size or by replacing an insert,such as a sleeved tube. Adjustments to the coolant flow through theorifices may be based on a one-time calibration using an infra-redcamera. The calibration can compensate for manufacturing defects in theESC and drastically improve the thermal performance characteristics ofthe cooling plate. The calibration process may use the measurements toadjust each orifice based on feedback from an infra-red camera. Thecooling plate may be measured and the orifices adjusted in an iterativeprocess until the desired heat distribution is obtained.

FIG. 5 is an isometric view of an ESC 500 showing a ceramic puck 502over a cooling plate 504. The diagram is simplified and does not includeelectrical, coolant and control connections. The diagram also does notshow a variety of different “feedthru” holes in the ceramic puck. Theseholes are required to accommodate gas outlets, temperature probes, andwafer lift pins.

FIG. 6 is an isometric view, partially cut-away of a system that isadapted particularly for measuring the thermal performance of a chuck,such as the ESC of FIG. 5. The system has a vacuum chamber 602 forholding the chuck 604 on a suitable base 614. The base holds the chuckabove the bottom of the chamber and provides an additional desiredsupport and equipment. It may also provide heating, and cooling,depending on the particular implementation. A heater 608 inside thechamber is used to heat the chuck. Shutters 610, between the heater andthe chuck, control when heat is applied to the chuck. A germanium window612 through the exterior wall of the vacuum chamber allows an infraredcamera 606 to measure the chuck temperature based on infrared radiation.These components are described in more detail below.

The vacuum chamber 602 is shown as a nearly cubic rectangular volumewith six walls at right angles and all about the same size. If thevacuum chamber is used at or near standard atmospheric pressure (atm),with typical processing chamber vacuum, the pressure on the outside ofthe chamber will not be great. A rectangular chamber may easily be madeto withstand many testing cycles. However, the chamber may be made inmany different shapes and configurations. The chamber shape may beadapted to improve heat flow or internal heat reflection. The interiorof the chamber may be coated or polished to reflect infrared radiation.In addition, the coating may be adapted to compensate for hot and coolareas in the chamber.

The test apparatus in the illustrated example includes upper 622 andlower 624 chamber halves. The two halves are connected by a hinge (notshown), and an o-ring (not shown) seals the two halves. The hinge allowsthe chamber to be opened to service components and to place the chuckthat is to be tested. The chamber can then be closed and latched shutfor another test.

A vacuum chamber is used for thermal testing for at least two reasons.First, the vacuum eliminates convective heat transfer. A partial orimperfect vacuum greatly reduces convective heat transfer. This allowsthe heat to be controlled more quickly. If the chuck is heated only orprimarily by infrared emission from the heater, then the heat can beturned on or off quickly. Air in the chamber would absorb heat. If theheater were turned off, then the heated air would continue to heat thechuck. Similarly if the heat is turned on, then the air would absorbsome of the heat. In both cases, the chuck responds more slowly tochanges in the heater.

A second reason for using a vacuum chamber is that wafers can bechucked, either electrostatically or by differential vacuum. Adifferential vacuum makes it easier to hold and move wafers. Adifferential vacuum hold does not add heat.

A third reason is that many fabrication processes are performed in avacuum. A vacuum chamber test will more closely resemble productionconditions.

The particular environment inside the chamber may be modified asdesired. For some thermal performance tests, a nitrogen or vaporenvironment may be preferred. In addition the quality or purity of thevacuum may be modified to suit particular testing purposes.

The chamber is cooled to reduce emission from other heat sources. Sincethe system is primarily intended to determine the thermal performance ofthe chuck, other heat sources within the chamber, including the chamberwalls, may interfere with the heat measurements. Liquid cooling may beused to maintain a constant and uniform chamber temperature. This may beused to improve the accuracy of the thermal imaging.

The infrared camera 606 makes the temperature measurements of the chuckduring tests. Any of a variety of different infrared cameras may beused. A commercially available camera such as a FLIR SC8000 withmoderately high resolution (1024×1024), a fast frame rate (130 Hz) andhigh sensitivity (0.015° C.) allows the chuck to be more accuratelycharacterized. The camera may be selected to meet the purpose of anyparticular testing system and testing purpose.

The germanium window 612 allows the camera to image the interior of thevacuum chamber from outside the vacuum chamber. For thermal imaging, thewindow is transparent to infrared radiation. While, the window may bemade of a variety of different materials that are transparent in theinfrared spectrum, germanium is readily available at reasonable cost.

The shutters 610 are formed from a set of plates that move between anopen position and a closed position. When closed, the shutters block thethermal radiation from the heater from reaching the chuck. When open,the shutters allow the heater to heat the chuck. The shutters allow thethermal radiation load to be quickly turned on and off, mimicking theconditions in an etch process chamber. A heater element will almostalways retain some residual heat slowing its response to on and offtransitions. The shutters allow the heat to be blocked or allowed. Theheater does not need to be switched the heat load on the chuck may becontrolled through the shutters. In one embodiment, the shutters may bemounted on axles, with the axles driven externally by rotary vacuumfeedthrus through the chamber walls.

Like the vacuum chamber walls, the shutters may be liquid cooled, inorder to maintain a consistent background for thermal imaging. Since theshutters block most or all of the thermal radiation from the heater whenclosed, they are exposed to a significant heat load from the heater.This may be reduced by coating the shutters in a reflective material onthe surfaces facing the heater. FIG. 6 shows the vacuum chamber with thechuck held in place near the bottom of the lower half of the chamber.The chuck is held apart from any wall of the chamber so that conductiveheat transfer with other surfaces does not affect the camera image. Thechuck is in the form of a wafer which is typically a round disk shape.The heater inside the chamber is mounted in the upper half of thechamber some distance from the chuck. In the illustrated example, theheater is an annulus above the chuck. This allows the camera to view thechuck through the central hole in the heater. The center of the chuckand the heater are aligned or close to aligned.

The shutters are directly below the heater, between the heater and thechuck. As a result, when the shutters are closed as shown in FIG. 6,there is a wall or thermal barrier horizontally across the interior ofthe chamber. The top of the chamber carrying the heater is walled off bythe shutters from the lower part of the chamber that has the chuck.

FIG. 7 shows the same chamber 602 with the same chuck 604 and heater608. In this case, the shutters are open. In the open position theshutters are dropped down away from the heater and there is no barrierbetween the heater and the chuck. In this example, the shutters are theshape of four triangular leaves. One side of each triangle is placedagainst one of the four chamber side wall. The opposite corners move upand down from vertical against the side walls to horizontal where thecorners meet in the middle of the chamber when the shutters are closed.Any of a variety of other shutter shapes may be used, depending on theparticular implementation.

Referring again to FIG. 6, the heater is in the form of an annulus. Theannular heater transfers a radiation heat load onto the ESC or wafer.The annulus provides for a uniform circular distribution of heatemission to coincide with the shape of the chuck. The central opening inthe annulus allows the camera to view the chuck through the heater. Thesize of the hole in the middle of the heater may be determined based onthe field of view of the camera lens, the relative distances of thecamera, heater, and chuck and the size of the chuck. The heater may bemade so that it has a diameter that is larger than the chuck and thatthe central opening is no larger than necessary to allow the camera toimage the whole top surface of the chuck.

The heater may be formed in any of a variety of different ways. In someembodiments, the heater may be formed from an array of heater filamentwire as shown in FIG. 8. FIG. 8 is an isometric bottom view of a heater808 that is formed based on a reflector dish 810. The reflector isformed from a rigid material that reflects or is coated to reflectinfrared radiation of the type produced by the heater. An aluminum platemay be used. The dish may be polished to increase the reflectance ofinfrared radiation. Since the heater and the chuck are operating in avacuum, the efficiency of the heater is improved by improving thetransmission of heat toward the chuck. In the same way, the interior ofthe vacuum chamber may be made reflective to infrared radiation in orderto enhance efficiency. This may be done by polishing, coating, painting,or in another way.

The reflector dish may be made from a single round plate of metal,composite, or other material with an appropriately sized hole throughthe middle. An array of radial standoffs 818 are mounted to the dish810. The array of heater filaments 812 are mounted onto electricallyinsulating standoffs. The position and spacing of the filaments is setto achieve a uniform thermal radiation on the chuck. As a result, theremay be more filaments toward the middle to compensate for the centralopening.

The filaments are shown as being in concentric circles and resting onthe standoffs. Alternatively, the filament may be a single spiral woundwire. In this example, the filaments act as resistive heaters thatgenerate infrared radiation when a current is driven through the wire.The filament may also generate other light, heat, and other products,but the infrared radiation is the primary product of interest for thisapplication. A wide variety of different filaments may be used thatgenerate infrared radiation in response to a current. The filaments maybe made of several isolated portions that are independently powered sothat the heat in different parts of the heater can be adjustedindependently.

In another embodiment, a cast heater plate may be used as shown in FIG.9. The heater plate 908 may be made to have a very uniform surfacetemperature by casting heater elements 912 within an aluminum disc 910.In both cases, the heater may be constructed with multiple independentzones. Each zone may then be electrically coupled to a controller thatwill independently switch the current and may also change the currentlevel to each zone independently. With such resistive heating elements,the heat output may be controlled by changing the amplitude or by pulsewidth modulation, depending on the nature of the controller and thetesting plan.

For the example of FIG. 8, a few heater filaments may be groupedtogether, and the heating duty cycle of each group may be controlledindividually. In the cast heater of FIG. 9, the heater elements 912 arein the form of concentric rings which may each be controlledindividually. A multi-zone heater approach allows careful control of theradial temperature of the heater, and would permit tuning of theradiation heat load to either uniform or non-uniform radial profiles asneeded. As an example, the inner heater elements might be driven to ahigher temperature than the outer heater elements in order to compensatefor the lack of heating in the middle of the heater. In anothersituation, there may be hot or cool spots at certain radial distancesfrom the center of the heater due to the shape of the vacuum chamber orthe design of the shutters.

In use, the vacuum chamber may be used for thermal testing of the chuckin a variety of different ways. In one example, the chuck is loaded intothe chamber at room temperature and ambient pressure. The chamber isevacuated using a vacuum pump. After or during evacuation, the heater isactivated. This causes the chuck to be heated by thermal radiation.After the chamber is evacuated and the interior of the chamber isprimarily a vacuum, the heater continues to heat the chuck. At the sametime, while the chuck is heating the temperature of the chuck ismeasured by thermal imaging. During a test, a thermal load can beapplied in a quick on-off cycle by opening and closing the shutters.This may be done to mimic the thermal conditions found in an etchprocess chamber. The camera may then observe changes in the surfacetemperature of the chuck during the thermal cycling. This testcapability provides dramatic improvements in ESC design andmanufacturing.

FIG. 10 is a process flow diagram illustrating a simplified set ofoperations to evaluate the thermal performance of a chuck using theapparatus described herein. At 10 a wafer chuck, such as but notnecessarily an electrostatic chuck, is placed on a base in a vacuumchamber. The wafer chuck has a top surface to hold a wafer. In somecases, a wafer may be attached to the chuck using electrostatic ofdifferential vacuum. The chuck may also be coupled to heat exchangersand power supplies for its heating and cooling system, if it has any.

At 11, the vacuum chamber is evacuated. A special process gas may beintroduced, depending on the particular test, or a simple vacuum may beused.

At 12, a heater is activated to heat the chuck. The heater may beactivated before the chamber is evacuated by the vacuum pump, or if theheating is slow, the heating may be started before the chamber iscompletely evacuated. For accurate thermal imaging, a vacuum willeliminate convective heat transfer. Accordingly, chuck performance isbetter evaluated with the vacuum. The heater may heat the chuck to atemperature of a plasma etch chamber to allow the performance of thechuck to be evaluated at working temperatures.

At 13, shutters are operated between the heater and the chuck. Theshutters restrict the flow of heat from the heater to the chuck. Whenthe shutters are closed, heat flow is restricted. When the shutters areopen heat flow is unrestricted or less restricted. The shutters areoperated during the measuring of the chuck temperature to simulatepulsing of a heat source in a plasma etch chamber.

At 14, the temperature of the top surface of the chuck is measured usingan infrared imager outside the vacuum chamber. While these operationsare shown as being sequential, they may overlap. As an example, thetemperature may be measured while the heater is operating, and theshutters are operating.

The chuck may be heated using an annular shaped heater with multipledifferent independently controllable heat zones. The heat zones may beconcentric about the annulus or in any other configuration depending onthe particular test. The heat produced by each of the heat zones can beindependently adjusted to mimic a particular plasma condition, performstress tests, uniformity tests, and even calibration tests.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, while flow diagrams inthe figures show a particular order of operations performed by certainembodiments of the invention, it should be understood that such order isnot required (e.g., alternative embodiments may perform the operationsin a different order, combine certain operations, overlap certainoperations, etc.). Furthermore, many other embodiments will be apparentto those of skill in the art upon reading and understanding the abovedescription. Although the present invention has been described withreference to specific exemplary embodiments, it will be recognized thatthe invention is not limited to the embodiments described, but can bepracticed with modification and alteration within the spirit and scopeof the appended claims. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

What is claimed is:
 1. An apparatus comprising: a vacuum test chamber; abase proximate the bottom of the chamber configured to support and holda wafer chuck; an annular heater above the chuck and electricallycoupled to an external power supply to heat the chuck by radiation, theannular heater having a central opening above the chuck; a window at thetop of the chamber to allow infrared radiation to pass through theexterior of the chamber; and an infrared imaging system in view of thewindow to receive infrared radiation from the chuck through the centralopening of the heater and to measure the temperature of the chuck. 2.The apparatus of claim 1, further comprising shutters between the heaterand the base, the shutters opening to allow heat to radiate from theheater to the chuck and closing to restrict heat radiation from theheater to the chuck.
 3. The apparatus of claim 1, wherein the shuttersare hinged against walls of the chamber to pivot with respect to thewalls to move up into a closed position to restrict heat radiation anddown away from the heater to an open position to allow heat radiation.4. The apparatus of claim 3, wherein the chamber is rectangular, whereina shutter is attached to each of four walls of the rectangular chamber,and wherein the four shutters close by moving a lower end toward thecenter of the chamber.
 5. An apparatus comprising: a chamber; a base tosupport a wafer chuck in the chamber; a heater to heat the chuck; awindow through the exterior of the chamber; and an infrared imagingsystem to measure the temperature of the chuck while the chuck isheated.
 6. The apparatus of claim 5, wherein the heater has a centralopening, wherein the heater is between the chuck and the imaging system,and wherein the imaging system measures the chuck through the centralopening.
 7. The apparatus of claim 6, wherein the heater is in the shapeof an annulus and wherein the central opening is in the center of theannulus.
 8. The apparatus of claim 5, wherein the chuck is circular,wherein the heater is circular, and wherein the heater has concentricheating zones that are controlled independently.
 9. The apparatus ofclaim 8, wherein the heater comprises a metal annulus with circular wireheating filaments.
 10. The apparatus of claim 5, further comprising acontroller to drive the heater to temperatures of a plasma etch chamber.11. The apparatus of claim 5, further comprising a vacuum pump toevacuate the chamber before heating the chuck.
 12. The apparatus ofclaim 5, wherein the chuck is an electrostatic chuck comprising heatingelements and coolant zones.
 13. The apparatus of claim 5, furthercomprising shutters between the heater and the base, the shuttersopening to allow heat to flow from the heater to the chuck and closingto restrict heat flow from the heater to the chuck.
 14. A methodcomprising: placing a wafer chuck on a base in a vacuum chamber, thewafer chuck having a top surface to hold a wafer; evacuating the vacuumchamber; heating the chuck; and measuring the temperature of the topsurface using an infrared imager outside the vacuum chamber.
 15. Themethod of claim 14, wherein heating the chuck comprises heating thechuck to a temperature of a plasma etch chamber.
 16. The method of claim14, further comprising operating shutters between the heater and thechuck during measuring the temperature to simulate pulsing of a heatsource in a plasma etch chamber.
 17. The method of claim 14, furthercomprising attaching a wafer to the top surface of the wafer chuckbefore placing the wafer chuck on the base.
 18. The method of claim 14,wherein heating the chuck comprises heating the chuck using radiant heatafter evacuating the vacuum chamber.
 19. The method of claim 18, whereinheating the chuck comprises heating the chuck with an annular shapedheater with a plurality of independently controllable heat zones. 20.The method of claim 14, wherein measuring the temperature comprisesimaging a thermal profile to the top surface of the chuck, the methodfurther comprising independently adjusting the heat radiated by the heatzones of the heater based on the thermal profile.